Plural stage purification of propylene oxide

ABSTRACT

Impure propylene oxide is purified by a distillation process wherein it is (a) extractively distilled in a first column using a C 2  to C 6  alkylene glycol extractive distillation agent to form a first overhead fraction comprising propylene oxide, C 5  -C 7  hydrocarbons methanol, water and oxygen-containing impurities, (b) wherein the first overhead fraction is separated in a second column into a second overhead fraction comprising most of the pentanes, pentenes and oxygen-containing impurities and a partially purified propylene oxide bottoms fraction comprising propylene oxide, hexenes, hexanes, and only residual quantities of pentenes and pentanes, (c) wherein the partially purified bottoms fraction is extracting distilled in a third column using a C 7  -C 10  alkane hydrocarbon extractive distillation agent to provide a further purified bottoms fraction containing substantially all of the propylene oxide, hexenes, hexanes charged to the third distillation column, and (d) the further purified bottoms fraction is extractively distilled in a fourth column using a C 7  to C 10  alkane hydrocarbon extractive distillation agent to provide a purified propylene oxide overhead fraction consisting essentially of propylene oxide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the plural stage purification of propyleneoxide. More particularly, this invention relates to a plural stagedistillation process for removing contaminating quantities of impuritiesfrom propylene oxide. Still more particularly, this invention relates toa plural stage distillation process for the purification of impurepropylene oxide contaminated with impurities including water, propylene,propane, acetaldehyde, methyl formate, propionaldehyde, hexenes,acetone, hexanes, methanol, t-butyl alcohol, pentane, isopentane,pentenes, isopropyl alcohol and t-butyl formate. The impure propyleneoxide is distilled by a plural stage process using a C₂ to C₆ alkyleneglycol extractive distillation agent and a C₇ to C₈ alkane hydrocarbonextractive distillation agent.

2. Prior Art

It is known to use alkane hydrocarbons containing 6 to 18 carbon atomsas extractive distillation agents in the purification of propyleneoxide. See, for example, Binning et al. U.S. Pat. No. 3,338,800, JubinU.S. Pat. No. 3,464,897, Jubin U.S. Pat. No. 3,607,669 and Schmidt U.S.Pat. No. 3,843,488. Jubin U.S. Pat. No. 3,464,897 shows that an alkanesuch as octane is effective for the removal of 6 carbon atom alkaneimpurities such as 2-methyl pentane, 4-methyl pentene-1, 2-methylpentene-1 and 2-methyl pentene-2. Schmidt U.S. Pat. No. 3,843,488 showsthat alkanes containing from 8 to 20 carbon atoms, and preferably from 8to 10 carbon atoms, such as n-octane, is effective for removinghydrocarbon impurities containing 5 to 7 hydrocarbons from propyleneoxide.

It is also known to use alkylene glycols containing from 2 to 6 carbonatoms as extractive distillation agents in the purification of propyleneoxide. See, for example, Washall U.S. Pat. No. 3,578,568, Kageyama etal. U.S. Pat. No. 3,838,020, Shih et al. U.S. Pat. No. 5,000,825,Marquis et al. U.S. Pat. No. 5,139,622, and Marquis et al. U.S. Pat. No.5,160,587. Thus, Washall U.S. Pat. No. 3,578,568 discloses the use ofethylene glycol and propylene glycol as extractive agents for thepurification of propylene oxide. Kageyama et al. U.S. Pat. No. 3,838,020discloses the use of butylene glycols for this purpose. Shih et al. U.S.Pat. No. 5,000,825 discloses the use of glycols containing 2 to 4 carbonatoms such as ethylene glycol, propane diol, butane diol, etc. Marquiset al. U.S. Pat. No. 5,139,622 discloses the use of triethylene glycolas an extractive distillation agent and Marquis et al. U.S. Pat. No.5,160,587 discloses the use of dipropylene glycol as an extractivedistillation agent.

The use of plural stage distillation for the purification of propyleneoxide has also been proposed.

Schmidt U.S. Pat. No. 3,881,996 discloses a plural stage process for thepurification of propylene oxide including a first fractionation whereinlight impurities such as acetaldehyde are removed overhead followed by asecond distillation step wherein heavy impurities such aspropionaldehyde are removed in order to provide a second distillatefraction which is then extractively distilled in the presence of octanein a third distillation zone to provide pure propylene oxide and toremove alkane hydrocarbon impurities such as C₆ carbon atom impuritiesfrom the propylene oxide. Schmidt et al. teach that it is important touse the proper sequence of distillation steps and that, for example,removal of heavy impurities such as propionaldehyde before the removalof light impurities such as acetaldehyde will lead to adverse results.

Shih U.S. Pat. No. 5,133,839 discloses a plural stage process for thepurification of propylene oxide utilizing a conventional distillationzone which preferably contains two distillation columns, as in SchmidtU.S. Pat. No. 3,881,996, followed by extractive distillation of theparts of purified propylene oxide in two sequential extractivedistillation columns using either isoctane or a lower alkylene glycol asthe extractive distillation agent and removing lighter impurities in thefirst of the extractive distillation columns and heavier impurities fromthe second of the extractive distillation columns.

Meyer et al. U.S. Pat. No. 4,971,661 discloses a plural stage processfor the purification of propylene oxide, but different extractivedistillation agents such as water and acetone are used.

BACKGROUND INFORMATION

As illustrated by the prior art just discussed, it was known prior tothe present invention to use plural stage distillation for thepurification of propylene oxide and to use lower alkylene glycols andhigher alkanes as extractive distillation agents. Thus, it was knownthat the use of the 8 to 10 carbon atom alkanes as extractivedistillation agents was particularly effective for removingcontaminating quantities of hydrocarbons such as hydrocarbons containing5 to 7 carbon atoms from propylene oxide. It was also known to use thelower alkylene glycols, which are polar solvents, for the extractivedistillation in order to remove oxygenated impurities.

Pentanes and pentenes are present in crude propylene oxide as minorimpurities. It has been discovered in accordance with the presentinvention that the removal of pentane and pentenes presents a difficultproblem.

Thus, as is pointed out, for example by Schmidt U.S. Pat. No. 3,881,996,the sequence of distillation steps employed in the plural stagepurification of propylene oxide is crucial to effective purification.Reversing or otherwise altering the sequence of steps can lead toadverse results.

Pentanes, isopentane and pentenes may be removed like other hydrocarbonimpurities by using octane in an extractive distillation. The octanelowers the relative volatility of the hydrocarbons relative to propyleneoxide. However, the degree of relative volatility change is proportionalto the amount of octane solvent present. Using typical solvent to feedratios as disclosed in the prior art and in this invention, it has beendiscovered that pentanes and pentenes are not easily removed and tend toremain in the PO product. Therefore, this invention discloses that amore economical and efficient means of removing pentanes and pentenes isby a conventional distillation upstream of the octane extractivedistillation. The C5 stripper tower should not be located downstream ofthe octan extractive distillation since the final PO product would thenbe a bottoms product off the stripper. It is advantageous to take highpurity products off the top of columns rather than off the bottom.

Thus, when partially purified propylene oxide containing contaminatingquantities of 5 to 7 carbon atom hydrocarbons such as pentanes,pentenes, hexanes, hexenes, heptanes and heptenes is subjected toextractive distillation using an alkane hydrocarbon extractivedistillation agent such as octane, a significant portion of the pentanesand pentenes will remain with the overhead purified propylene oxiderather than being removed therefrom for withdrawal from the bottom ofthe tower together with the extractive distillation agent and the otherimpurities. As a consequence, a subsequent distillation step wouldnormally be required for the removal of the pentanes and pentenes.

It has been discovered in accordance with the present invention,however, that the use of a different sequence of distillation steps fromthat disclosed in the prior art will obviate this problem so that thereis no need for an additional distillation tower downstream of the alkanehydrocarbon extractive distillation.

SUMMARY OF THE INVENTION

In accordance with the present invention, impure propylene oxide isseparated from contaminating quantities of hexanes, hexenes, pentanes,pentenes, and oxygen-containing impurities by a sequential extractivedistillation process wherein the impure propylene oxide is firstextractively distilled using a lower alkylene glycol extractivedistillation agent to form a first overhead fraction comprisingpropylene oxide, hexanes, hexenes, pentanes and pentenes andoxygen-containing impurities boiling above propylene oxide, wherein thefirst overhead distillation fraction is separated in a seconddistillation column into a second overhead distillation fractioncomprising most of the pentanes, pentenes, oxygen-containing impuritiesboiling above propylene oxide and a residual amount of propylene oxideand a partially purified propylene oxide bottoms fraction comprisingpropylene oxide, hexenes, hexanes and only residual quantities ofpentenes and pentanes, wherein the partially purified bottoms fractionis extractively distilled in a third distillation column using a C₇ -C₈alkane hydrocarbon extractive distillation agent to provide a furtherpurified bottoms fraction containing substantially all of the propyleneoxide, hexenes, hexanes and pentenes charged to the third distillationcolumn and wherein the partially purified propylene oxide bottomsfraction is further purified in an extractive distillation column usinga C₇ -C₈ alkane hydrocarbon extractive distillation agent to provide apure propylene oxide overhead fraction consisting essentially ofpropylene oxide.

In accordance with a preferred embodiment of the present invention,impure propylene oxide contaminated with pentanes, pentenes, hexenes,hexanes and oxygen-containing impurities is purified by a sequentialextractive distillation process wherein (a) the impure propylene oxideis extractively distilled using a first C₂ to C₆ alkylene glycolextractive distillation agent to form a first lighter distillationfraction containing all of the propylene oxide, hexenes, hexanes,pentenes and pentanes and oxygen-containing impurities boiling abovepropylene oxide and a first heavier distillation fraction containingwater and oxygen-containing impurities boiling below propylene oxide andall of the first extractive distillation agent, wherein (b) the firstlighter distillation fraction is separated in a second distillationcolumn into a second lighter distillation fraction containing most ofthe pentanes, pentenes, oxygen-containing impurities boiling abovepropylene oxide and a residual amount of propylene oxide and a secondheavier distillation fraction containing substantially all of thepropylene oxide, hexenes, hexanes and only residual quantities ofpentenes and pentanes, wherein (c) the second heavier distillationfraction is extractively distilled using a C₇ to C₈ alkene hydrocarbonextractive distillation agent to provide a third lighter distillationfraction containing residual oxygen-containing impurities and water anda third heavier distillation fraction containing substantially all ofthe propylene oxide, hexenes, hexanes and residual pentanes in anextractive distillation agent and wherein (d) the third heavierdistillation fraction is extractively distilled using a C₇ to C₈ alkaneextractive distillation agent to provide a fourth lighter distillationfraction consisting essentially of pure propylene oxide and a fourthheavier distillation fraction containing all of the extractivedistillation agent, all the hexenes, hexanes and also residualquantities of propylene oxide, pentenes and pentanes.

In accordance with the preferred embodiment of the present invention, adistillation process is provided for the purification of impurepropylene oxide contaminated with water, propylene, propane,acetaldehyde, methyl formate, propionaldehyde, hexenes, acetone,hexanes, methanol, t-butyl alcohol, pentane, isopentane, pentenes,isopropyl alcohol and t-butyl formate comprising the steps of:

a. Charging the impure propylene oxide to a first extractivedistillation column;

b. Charging a lower alkylene glycol extractive distillation agent to thefirst extractive distillation column at a feed point above the point ofintroduction of the impure propylene oxide;

c. Fractionating the impure propylene oxide in the first extractivedistillation column under distillation conditions selected to provide afirst lighter distillation fraction containing all of the propyleneoxide, propylene, propane, acetaldehyde, methyl formate, hexenes,hexanes, pentenes and pentanes and a portion of the methanol and waterand also a first heavier distillation fraction containing all of thepropionaldehyde, acetone, t-butyl alcohol, t-butyl formate, isopropylalcohol and the remainder of the methanol and water and all of theextractive distillation agent;

d. Charging a second feedstock comprising the first lighter distillationfraction to a second distillation column;

e. Fractionating the second feedstock in the second distillation columnunder distillation conditions selected to provide a second lighterdistillation fraction containing all of the propylene, propane,acetaldehyde, pentanes, pentenes and a residual amount of propyleneoxide and a second heavier distillation fraction containingsubstantially all of the propylene oxide, hexenes, hexanes and only aresidual quantity of pentenes and pentanes;

f. Charging a third feedstock comprising the heavier distillationfraction to a third extractive distillation column;

g. Charging a C₇ to C₈ alkane hydrocarbon extractive distillation agentto the third extractive distillation column at a feed point above thepoint of introduction of the third feedstock;

h. Fractionating the third feedstock in the third extractivedistillation column under distillation conditions selected to provide athird lighter distillation fraction containing all of the acetaldehyde,methyl formate, methanol, water and residual quantities of propyleneoxide and hexane and a third heavier distillation fraction containingsubstantially all of the propylene oxide and extractive distillationagent charged to the third distillation column in all of the hexenes andhexanes charged to the third distillation column;

i. Charging a fourth feedstock comprising the third heavier distillationfraction to a fourth extractive distillation column;

j. Charging a C₇ to C₈ alkene hydrocarbon extractive distillation agentto the fourth extractive distillation column at a feed point above thepoint of introduction of the third feedstock; and

k. Fractionating said fourth feedstock in the fourth extractivedistillation column under distillation conditions selected to provide afourth lighter distillation fraction consisting essentially of purepropylene oxide, and a fourth heavier distillation fraction containingsubstantially all of the extractive distillation agent, all the hexenesand hexanes and also the residual quantities of propylene oxide,pentenes and pentanes

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic flow sheet with conventional parts omittedshowing the general recovery sequence that is used in accordance withthe present invention to purify propylene oxide.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawing, there is shown a schematic flow sheetillustrating a preferred method for practicing the process of thepresent invention. In the drawing, conventional parts such as valves,pumps, temperature sensors, pressure sensors, heaters, coolers, flowcontrol regulation apparatus, reboilers, reflux condensers, etc., havebeen omitted.

The impure propylene oxide to be purified in accordance with the presentinvention is typically propylene oxide prepared by the reaction oftertiary butyl hydroperoxide with propylene in the presence of amolybdenum catalyst to provide a reaction mixture comprising unreactedpropylene, unreacted tertiary butyl hydroperoxide, tertiary butylalcohol, propylene oxide, and impurities. This reaction product isseparated in a distillation zone (not shown) into a plurality offractions including a propylene recycle fraction, an impure propyleneoxide fraction, a tertiary butyl alcohol fraction and a residuefraction.

The impure propylene oxide obtained in this fashion is suitably used asa feedstock for the present invention and will normally be contaminatedwith impurities including water, propylene, propane, acetaldehyde,methyl formate, propionaldehyde, hexenes, acetone, hexanes, methanol,tertiary butyl alcohol, pentane, isopentane (i.e., crude ash methylbutane), pentenes, isopropyl alcohol and tertiary butyl formate.

The impure propylene oxide will normally contain from about 97 to about99 wt.% of propylene oxide, the balance being impurities such as thoseenumerated above. It will be understood that some of the enumeratedimpurities will not always be present in impure propylene oxide and thatother impurities not listed may be present in minor quantities. Theimpurities, broadly speaking, comprise water, hydrocarbons such aspropylene, propane, hexenes, hexanes, pentane, isopentane and pentenes,and oxygenated impurities including aldehydes, alcohols, esters, etc.The hydrocarbon impurities will normally contain from about 2 to about 6carbon atoms and the oxygenated impurities will normally contain fromabout 2 to about 6 carbon atoms also.

In accordance with the present invention, an impure propylene oxide ofthe type described is charged to a first extractive distillation column10 by way of a charge line 12. The fractional distillation column 10 maysuitably comprise from about 40 to about 80 theoretical trays and thecharge line 12 for the impure propylene oxide will normally be at leastabout 10 to about 30 trays from the bottom of the distillation tower.

An alkylene glycol extractive distillation agent is also charged to theline 10 by a line 14. The alkylene glycol extractive distillation agentmay suitably be an alkylene glycol containing from 2 to 6 carbon atomssuch as ethylene glycol, propylene glycol, 1,4-propane diol,1,3-2-methyl propane diol, 1,4-butane diol, 2-methyl-1,3-propane diol,diethylene glycol, triethylene glycol, dipropylene glycol, etc.Preferred extractive distillation agents include triethylene glycol anddipropylene glycol.

The alkylene glycol extractive agent 14 will suitably be charged to theextractive distillation column 10 in the ratio of about 2 to 7 parts ofimpure propylene oxide per part of alkylene glycol extractivedistillation agent.

Extractive distillation conditions are adjusted in the extractivedistillation tower 10 so as to provide for the recovery overhead or as adistillate fraction of substantially all of the propylene oxide chargedto the extractive distillation column 10. For example, the impurepropylene oxide, which will suitably comprise about 98.5 wt.% propyleneoxide, the balance being impurities as mentioned above, may be chargedat a temperature of about 100° F. to about 160° F.

The temperature at the bottom of the extractive distillation column 10may suitably be about 300°-380° F. and the pressure may be about 20-50psia. The temperature at the top of the extractive distillation column10 may, for example, be about 90°-130° F. and the pressure may suitablybe about 10-30 psia. A first lighter distillation fraction comprisingimpure propylene oxide is removed from the column 10 by way of a line 16and may suitably comprise more than 99 wt.% propylene oxide, the balancebeing impurities including acetaldehyde, methyl formate, hexenes,hexanes, methanol, water, pentanes, isopentanes and pentenes. A heavierdistillation fraction 18 discharged adjacent the bottom of theextractive distillation column 10 will comprise impurities includingwater, propionaldehyde, acetone, methanol, tertiary butyl alcohol,isopropyl alcohol, tertiary butyl formate and the alkylene glycolextractive distillation agent.

A second feedstock comprising the first lighter distillation fraction 16is charged to a second distillation column 20 which may suitably containfrom about 20 to about 40 theoretical trays, the second feedstock beingsuitably charged to about 8 to about 16 theoretical trays from thebottom of the tower. Distillation conditions are adjusted in thedistillation column 20 to provide a second lighter distillation fractiondischarged by way of a line 22 and a second heavier distillationfraction discharged by a line 24. Distillation conditions are suitablyadjusted in the distillation column 20 so that substantially all of thepropylene oxide will be present in the second heavier distillationfraction 24. Typically, the lighter distillation fraction 22 willcomprise impurities including propylene, propane, acetaldehyde and,significantly, substantially all of the pentanes and pentenes charged tothe distillation column 20. The second heavier distillation fractionwill typically contain substantially all of the propylene oxide,hexenes, hexanes and only residual quantities of pentanes and pentenes.

A third feedstock comprising the second heavier distillation fraction 24is charged to a third distillation column 30 together with a C₇ to C₈alkane hydrocarbon extractive distillation agent which is charged by aline 32.

The alkane extractive distillation agent may suitably be charged to thethird distillation column 30 in the ratio of about 5 to about 7 parts ofextractive distillation agent per part of third feedstock 24. Extractivedistillation conditions are adjusted in the distillation column 30 toprovide for a fourth heavier distillation fraction 33 containingsubstantially all of the propylene oxide, hexenes, hexanes, and residualpentenes and pentanes and a lighter distillation fraction 34 which willtypically comprise impurities such as methyl formate, acetaldehyde,water and methanol. Residual quantities of propylene oxide and hexanemay also be present.

The extractive distillation agent charged to the distillation column 30is suitably an 8 carbon atom alkane such as normal octane or isooctane.A minor amount of an alkane such as nonane (usually less than 1 wt.%)may also be present in the extractive distillation agent charged to theextractive distillation tower 30 by the line 32. The third feedstock 24charged to the third distillation column 30 may be suitably charged at atemperature of about 130°-180° F. The temperature at the bottom of theextractive distillation column 30 may suitably be about 180°-220° F. andthe pressure may suitably be about 30-50 psia. The temperature adjacentthe top of the extractive distillation tower 30 may suitably be about130°-160° F. and the pressure may suitably be about 20-40 psia.

A fourth feedstock comprising the third heavier distillation fraction 33is charged to a fourth extractive distillation tower 40 together with analkane extractive distillation agent such as octane which is charged byway of a line 42. The extractive distillation agent 42 may be charged tothe extractive distillation column 40 in the ratio of about 0.4 to about0.8 parts of extractive distillation agent per part of fourth feedstock33.

Distillation conditions are adjusted in the distillation column 40 toprovide lighter distillation fraction 44 consisting essentially ofpropylene oxide and a heavier distillation fraction 46 comprising thehexenes, hexanes, residual pentenes, residual pentanes and theextractive distillation agent.

Suitably, the temperature at the top of the distillation column 40 maybe about 100°-130° F. and the pressure may be about 10-30 psia. Thetemperature adjacent the bottom of the distillation column 40 maysuitably be about 250°-290° F. and the pressure may suitably be about25-45 psia.

A fifth feedstock comprising the first heavier distillation fraction 18from the first distillation column 10 may be charged to a fifthdistillation column 50 containing from about 20 to about 60 theoreticaltrays wherein the fifth feedstock may be separated under distillationconditions adjusted to provide for the recovery of substantially all ofthe alkylene glycol extractive distillation agent as a heavierdistillation fraction discharged from the distillation column by a line14 and a lighter distillation fraction 54 discharged from thedistillation column 50 and suitably containing substantially all of thewater, acetone, tertiary butyl alcohol, methanol and isopropyl alcoholcontained in the fifth feedstock 18.

The pressure at the top of the fifth distillation column 50 may suitablybe about 0.1-1.0 psia and the temperature may suitably be about 90°-130°F. The pressure at the bottom of the distillation column 50 may suitablybe about 0.1-1.1 psia and the temperature may suitably be about300°-360° F.

A sixth feedstock comprising the third lighter distillation fraction 34and the fourth heavier distillation fraction 46 may suitably be chargedby way of a line 48 to a sixth distillation column 60 which maycomprise, for example, from about 30 to about 50 theoretical trays andwherein the sixth feedstock is introduced at least about 20 theoreticaltrays from the bottom of the column. Distillation conditions areadjusted within the distillation column 60 to provide for the recoveryof a sixth lighter distillation fraction 62 comprising acetaldehyde,methyl formate, propylene oxide, hexenes, methanol and a minor amount ofoctane and a sixth heavier distillation fraction 64 comprisingsubstantially all of the extractive distillation agent charged to thedistillation column 60 by the line 48. For example, the distillationconditions established in the sixth distillation column 60 may include apressure at the top of the column of about 15-30 psia and a temperatureof about 100°-140° F. and a pressure at the bottom of the column ofabout 20-40 psia and a temperature of about 200°-300° F.

EXAMPLES

The invention will be further illustrated by the following specificexample which is given by way of illustration and not as a limitation onthe scope of this invention. Where parts are mentioned, they are partsby weight.

About 1000 parts of an impure propylene oxide feedstock 12 containingabout 98.5 wt.% of propylene oxide and contaminated with impuritiesincluding water, propylene, propane, acetaldehyde, methyl formate,propionaldehyde, hexenes, acetone, hexanes, methanol, t-butyl alcohol,n-pentane, isopentane (i.e., 2-methyl butane), pentene, isopropylalcohol and tertiary butyl formate and separated under the extractivedistillation conditions in the presence of a triethylene glycolextractive distillation agent into a first lighter distillation fraction16 (about 986 parts) containing more than about 99 wt.% of propyleneoxide and less than about 1 wt.% of contaminants including propylene,propane, acetaldehyde, methyl formate, hexenes, hexanes, pentenes,pentanes, water and methanol and a first heavier distillation fractioncontaining water, propionaldehyde, acetone, tertiary butyl alcohol,tertiary butyl formate, isopropyl alcohol, methanol and the alkyleneglycol and the triethylene glycol extractive distillation agent. Thefirst heavier distillation fraction 18 is separated in fifthdistillation column 50 into a heavier fraction 14 comprising thetriethylene glycol extractive distillation agent which is recycled tothe extractive distillation column 10 and a lighter overhead fraction 54comprising water, acetone, tertiary butyl alcohol, methanol andisopropyl alcohol.

A second feedstock comprising the lighter distillation fraction 16 isseparated in the second distillation column 20 into about 985 parts of aheavier distillation fraction 24 and about 1 part of a lighterdistillation fraction 22 containing propylene, propane, acetaldehyde,pentenes and pentanes charged to the extractive distillation column 10by the line 12.

A second heavier distillation fraction 24 discharged from thedistillation column 20 will comprise substantially all of the propyleneoxide, hexenes, hexanes and only residual quantities of pentanes andpentenes. This fraction is charged by way of the line 24 to a thirddistillation column 30 together with about 5620 parts of octane chargedto the distillation column 30 by a line 32. The distillation column 30is operated in the manner described above to provide a lighterdistillation fraction 34 (about 1 part) and a heavier distillationfraction 33 (about 6604 parts). The fraction 34 will compriseacetaldehyde, methyl formate, methanol, water and residual quantities ofpropylene oxide and hexane. The third heavier distillation fraction 33will comprise propylene oxide and substantially all of the octanecharged to the third distillation column 30.

A fourth feedstock comprising a third heavier distillation fraction 33is charged to the fourth extractive distillation column 40 where it isseparated together with about 3690 parts of octane in the mannerdescribed above into about 985 parts of an overhead distillationfraction consisting essentially of propylene oxide and containing verylow quantities of the impurities present in the feedstock charged by theline 12 to the first distillation column 10. A heavier distillationfraction 46 discharged from the column 40 will comprise about 9309 partsof octane, hexenes, hexanes and residual quantities of propylene oxide.

Having thus described our invention, what is claimed is:
 1. A processfor separating contaminating quantities of hexenes, hexanes, pentenes,pentanes, water and oxygen-containing impurities from impure propyleneoxide which comprises:extractively distilling the impure propylene oxidein a first distillation column using a C₂ to C₆ alkylene glycolextractive distillation agent to form a first overhead fractioncomprising propylene oxide, hexenes, hexanes, pentenes, pentanes, water,methanol and oxygen-containing impurities boiling above propylene oxide;fractionating the first overhead fraction in a second distillationcolumn to obtain a second overhead fraction comprising essentially allof the pentanes and pentenes and most of the oxygen-containingimpurities boiling above propylene oxide and a partially purifiedpropylene oxide bottoms fraction comprising propylene oxide, hexenes,hexanes, and only residual quantities of pentenes and pentanes;extractively distilling the partially purified bottoms fraction in athird distillation column using a C₇ to C₁₀ alkane hydrocarbonextractive distillation agent to provide a further purified bottomsfraction comprising substantially all of the propylene oxide, hexenesand hexanes charged to the third distillation column; and extractivelydistilling the further purified bottoms fraction in a fourthdistillation column using a C₇ to C₁₀ alkane hydrocarbon extractivedistillation agent to provide a purified propylene oxide overheadfraction consisting essentially of propylene oxide free of saidcontaminants.
 2. A distillation process for the purification of animpure propylene oxide contaminated feedstock with hexenes, hexanes,pentanes, pentenes, water and oxygen-containing impurities whichcomprises the steps of:separately charging said impure propylene oxidefeedstock and a C₂ to C₆ alkylene glycol extractive distillation agentto a first extractive distillation column at ascending charge points,and separating said impure propylene oxide therein into a first lighterdistillation fraction containing residual methanol and water and all ofthe propylene oxide, hexenes, hexanes, pentenes and pentanes andoxygen-containing impurities boiling with and above propylene oxide anda first heavier distillation fraction containing oxygen-containingimpurities boiling below propylene oxide and all of the extractivedistillation agent, charging a second feedstock comprising said firstlighter distillation fraction to a second distillation column andseparating it therein into a second lighter distillation fractioncontaining most of the pentanes and pentenes, oxygen-containingimpurities boiling above propylene oxide, and a second heavierdistillation fraction containing substantially all of the propyleneoxide, methanol, water, hexenes, hexanes, and only residual quantitiesof pentenes and pentanes, separately charging a third feedstockcomprising said second heavier distillation fraction and a C₇ to C₁₀alkane hydrocarbon extractive distillation agent to a third extractivedistillation column at ascending charge points and fractionating themtherein into a third lighter distillation fraction containing methanol,water and oxygen-containing impurities boiling above propylene oxide,and a third heavier distillation fraction containing substantially allof the propylene oxide, hexenes and hexanes, and extractive distillationagent charged to said third distillation column, separately charging afourth feedstock comprising said third heavier distillation fraction anda C₇ to C₁₀ alkane hydrocarbon extractive distillation agent to a fourthextractive distillation column at ascending charge points andfractionating it therein into a fourth lighter distillation fractionconsisting essentially of propylene oxide and a fourth heavierdistillation fraction containing substantially all of the extractivedistillation agent, all of the hexenes and hexanes charged to said thirddistillation column, and also residual quantities of propylene oxide. 3.A distillation process for the purification of an impure propylene oxidecontaminated feedstock with impurities including water, propylene,propane, acetaldehyde, methyl formate, propionaldehyde, hexenes,acetone, hexanes, methanol, t-butyl alcohol, pentane, isopentane,pentenes, isopropyl alcohol and t-butyl formate which comprises thesteps of:charging said impure propylene oxide feedstock to a firstextractive distillation column, charging a C₂ to C₆ alkylene glycolextractive distillation agent to said first extractive distillationcolumn at a feed point above the point of introduction of said impurepropylene oxide, fractionating said impure propylene oxide in said firstextractive distillation column under distillation conditions selected toprovide a first lighter distillation fraction containing all of thepropylene oxide, propylene, propane, acetaldehyde, methyl formate,hexenes, hexanes, pentenes and pentanes, and a portion of the methanoland water and a first heavier distillation fraction containing all ofthe propionaldehyde, acetone, t-butyl alcohol, t-butyl formate,isopropyl alcohol, the remainder of the methanol and water and all ofthe extractive distillation agent, charging a second feedstockcomprising said first lighter distillation fraction to a seconddistillation column, fractionating said second feedstock in said seconddistillation column under distillation conditions selected to provide asecond lighter distillation fraction containing all of the propylene,propane, acetaldehyde, pentanes and pentenes, and a residual amount ofpropylene oxide, and a second heavier distillation fraction containingsubstantially all of the propylene oxide, hexenes, hexanes, and onlyresidual quantities of pentenes and pentanes, charging a third feedstockcomprising said second heavier distillation fraction to a thirdextractive distillation column, charging a C₇ to C₁₀ alkane hydrocarbonextractive distillation agent to said third extractive distillationcolumn at a feed point above the point of introduction of said thirdfeedstock, fractionating said third feedstock in said third extractivedistillation column under distillation conditions selected to provide athird lighter distillation fraction containing all of the acetaldehyde,methyl formate, methanol and water and residual quantities of propyleneoxide and octane, and a third heavier distillation fraction containingsubstantially all of the propylene oxide and extractive distillationagent charged to said third distillation column and all of the hexenesand hexanes charged to said third distillation column, charging a fourthfeedstock comprising said third heavier distillation fraction to afourth extractive distillation column, charging a C₇ to C₁₀ alkanehydrocarbon extractive distillation agent to said fourth extractivedistillation column at a feed point above the point of introduction ofsaid third feedstock, fractionating said fourth feedstock in said fourthextractive distillation column under distillation conditions selected toprovide a fourth lighter distillation fraction consisting essentially ofpropylene oxide and a fourth heavier distillation fraction containingsubstantially all of the extractive distillation agent, all of thehexenes and hexanes charged to said third distillation column, and alsoresidual quantities of propylene oxide.
 4. A method as in claim 3wherein the pressure maintained at the top of the first distillationcolumn is about 10-30 psia, wherein the temperature maintained at thetop of the first distillation column is about 90°130° F., wherein thepressure maintained at the bottom of the first distillation column isabout 20-50 psia, and wherein the temperature maintained at the bottomof the first distillation column is about 300°-380° F.
 5. A method as inclaim 3 wherein the pressure maintained at the top of the seconddistillation column is about 35-45 psia, wherein the temperaturemaintained at the top of the second distillation column is about110°-130° F., wherein the pressure maintained at the bottom of thesecond distillation column is about 40-50 psia, and wherein thetemperature maintained at the bottom of the second distillation columnis about 150°-160° F.
 6. A method as in claim 3 wherein the pressuremaintained at the top of the third distillation column is about 20-40psia, wherein the temperature maintained at the top of the thirddistillation column is about 130°-160° F., wherein the pressuremaintained at the bottom of the third distillation column is about 30-50psia, and wherein the temperature maintained at the bottom of the thirddistillation column is about 180°-220° F.
 7. A method as in claim 3wherein the pressure maintained at the top of the fourth distillationcolumn is about 10-30 psia, wherein the temperature maintained at thetop of the fourth distillation column is about 100°-130° F., wherein thepressure maintained at the bottom of the fourth distillation column isabout 25-45 psia, and wherein the temperature maintained at the bottomof the fourth distillation column is about 250°-290° F.
 8. A method asin claim 3 wherein the first distillation column contains about 40 toabout 80 theoretical trays, wherein the first feedstock is introduced atleast about 10 trays from the bottom of the column, wherein the firstextractive distillation agent is introduced at least about 30theoretical trays above the tray at which the first feedstock isintroduced, and wherein the weight ratio of first feedstock to the firstextractive distillation agent is about 2 to about
 7. 9. A method as inclaim 3 wherein the second distillation column contains about 20 toabout 40 theoretical trays, and wherein the second feedstock isintroduced at least about 8 trays from the bottom of the column.
 10. Amethod as in claim 3 wherein the third distillation column containsabout 50 to about 80 theoretical trays, wherein the third feedstock isintroduced at least about 30 trays from the bottom of the column,wherein the third extractive distillation agent is introduced at leastabout 10 theoretical trays above the tray at which the third feedstockis introduced, and wherein the ratio of third feedstock to the thirdextractive distillation agent is about 0.1 to about 0.3.
 11. A method asin claim 3 wherein the fourth distillation column contains about 40 toabout 80 theoretical trays, wherein the fourth feedstock is introducedat least about 3 trays from the bottom of the column, wherein the fourthextractive distillation agent is introduced at least about 15theoretical trays above the tray at which the fourth feedstock isintroduced, and wherein the ratio of fourth feedstock to the fourthextractive distillation agent is about 1.3 to about 2.2.
 12. A method asin claim 3 wherein a fifth feedstock comprising said first heavierdistillation fraction is charged to a fifth distillation columnwherein:said fifth feedstock is fractionated in said fifth distillationcolumn under distillation conditions selected to provide a fifth lighterdistillation fraction containing substantially all of the water,acetone, t-butyl alcohol, methanol and isopropyl alcohol charged to thefifth distillation column and a fifth heavier distillation fractioncontaining substantially all of the alkylene glycol extractivedistillation agent charged to the fifth distillation column, and whereinsaid fifth heavier distillation fraction is recycled to said firstdistillation column as said first extractive distillation agent.
 13. Amethod as in claim 12 wherein the pressure maintained at the top of thefifth distillation column is about 0.1-1.0 psia, wherein the temperaturemaintained at the top of the fifth distillation column is about 90°-130°F., wherein the pressure maintained at the bottom of the fifthdistillation column is about 0.1-1.1 psia, and wherein the temperaturemaintained at the bottom of the fifth distillation column is about300°-360° F.
 14. A method as in claim 13 wherein the fifth distillationcolumn contains about 10 to about 30 theoretical trays, and wherein thesixth feedstock is introduced at least about 10 trays from the bottom ofthe column.
 15. A method as in claim 3 wherein a sixth feedstockcomprising said third lighter distillation fraction and said fourthheavier distillation fraction is charged to sixth distillation column,and whereinsaid sixth feedstock is fractionated in said sixthdistillation column under distillation conditions selected to provide asixth lighter distillation fraction containing substantially all of theacetaldehyde, methyl formate, propylene oxide, hexenes, methanol, waterand a minor portion of the octane charged to the sixth distillationcolumn and a sixth heavier distillation fraction containingsubstantially all of the C₇ to C₈ alkane extractive distillation agentcharged to the third and fourth distillation columns, and wherein saidsixth heavier distillation fraction is recycled to said third and fourthdistillation columns as said third and fourth extractive distillationagents.
 16. A method as in claim 15 wherein the pressure maintained atthe top of the sixth distillation column is about 15-30 psia, whereinthe temperature maintained at the top of the sixth distillation columnis about 110°-140° F., wherein the pressure maintained at the bottom ofthe sixth distillation column is about 20-40 psia, wherein thetemperature maintained at the bottom of the sixth distillation column isabout 200°-300° F.
 17. A method as in claim 15 wherein the sixthdistillation column contains about 30 to about 50 theoretical trays, andwherein the sixth feedstock is introduced at least about 20 trays fromthe bottom of the column.